Motor control system

ABSTRACT

In a direct current drive system for an elevator, a high-gain amplifier controls the speed of the motor through variable energization of the generator field in accordance with an error signal developed as a function of the difference between a signal developed by a pattern generator and a signal developed by a tachometer. Switchless feedback circuits are provided to bring the car smoothly to a creep speed should the pattern generator or tachometer generator fail as an open circuit. A noncontinuous feedback circuit and variable damping on the pattern generator minimize jerk on one floor runs but do not interfere with leveling. Should the pattern generator fail to initiate deceleration by a predetermined point as the car approaches a terminal, a fixed low speed signal brings the car to the landing. When the car is operating on reduced pattern signals, normal door preopening is discontinued.

United States Patent 2,667,610 1/1954 Schmitt et al 72] InventorsWilliam R. Caputo 318/295 X w k ff; 2,743,408 4/1956 Schmitt 318/295 Xglllllinlm M. Ostrander, Hackensack, both Primary Examiner oris L RadarAssistant Examiner-W. E. Duncanson, Jr. [21] P 845604 Attorneys-A. T.Stratton, C. L. Freedman and Richard V. [22] Filed June 30, 1969Westerhoff I [45] Patented Aug. 17, 1971 [73] Assignee WestinghouseElectric Corporation Pittsburgh, ABSTRACT: In a direct current drivesystem for an elevator. 11

high-gain amplifier controls the speed of the motor through variableenergization of the generator field in accordance with 154} MOTORCONTROL SYSTEM an error signal developed as a function of the differencen Claims, 2 Drawing Figs between a signal developed by a patterngenerator and a signal developed by a tachometer. Switchless feedbackcircuits are [52] US. (,l 187/29 R provided to bring the car smoothly ma Creep Speed Should the [SI Int. Cl B66b l/30 pattern generator ortachometer generator f il as an open [50] Field of Search 187/29; cuiL Anoncominuous f db k circuit and variable damping 313/20 745, 145, on thepattern generator minimize jerk on one floor runs but 317, 329, 386, 449do not interfere with leveling. Should the pattern generator fail toinitiate deceleration by a predetermined point as the car [56]References cued approaches a terminal, a fixed low speed signal bringsthe car UNITED STATES PATENTS to the landing. When the car is operatingon reduced pattern 1,962,344 6/1934 Geiselman 318/345 X signals, normaldoor preopening is discontinued.

PATTERN GENERATOR ZDI 202 R l 07 I w: c8 R32 1 R35 [1 n n I I i 281 B-21 S I 75 R272 I SUPERVISORY 2 A I CONTROL SCR AMPLIFIER -GDI I R29 =cH F5 I we an Ac RECTIFIER E l C6 /CL-7 R3l d 7\ 1 R28 I I 4 R30 1 2-6 I 7HAND 97 59-" CONTROL r- I FULL ,-R25 UL HOV WAVE k IUUL 5 R36 9 I ACRECTIFIER BJ A 77 M 1, i s-z; BIAS 6| UPPER TERMINAL 57 TSD-l WWW 3J- Us69 C\65 INTERMEDIATE LANDING D50 LOWER TERMINAL CIRCUITS DIRECTIONPATENTED AUG 1 7 m7:

SAFETY CIRCUITS TSD JAE-2 our .5 SEC DELAY fTSD-Z GD (EVA-3 [SEC DELAY55'2 gCLT-I PHL'I GD DSDW W TS D-3 7 USD d' MOTOR CONTROL SYSTEMCROSS-REFERENCE TO RELATED APPLICATIONS The commonly owned applicationof William R. Caputo, Ser. No. 837,442, entitled Motor Control Systemfiled concurrently herewith, relates to the basic motor control systemupon which the subject matter of this application operates. In order tominimize the complexity of this application, a detailed description ofonly those portions of the overall system necessary for an understandingof the subject matter of this application will be given. In order tocomplete the detailed description of the overall invention, theabove-mentioned application of Caputo is hereby incorporated byreference into this application.

For the purpose of fully disclosing a working embodiment of the system,the application of Andrew F. Kirsch entitled Pulse-SupervisedTransportation Systems, Ser. No. 606,239, filed Dec. 30, 1966 now US.Pat. No. 3,519,106 is also incor porated by reference into thisapplication for the purpose of disclosing a supervisory system whichgenerates certain control signals required in the operation of thepreferred embodiment of the invention described. In addition, theapplication of Conwell Savage entitled Motor Control Mechanism, Ser. No.817,789 filed on Apr. 21, 1969 and assigned to the same assignee ishereby incorporated by reference into this application for the purposeof disclosing a suitable pattern generator. Furthermore, the applicationof William R. Caputo entitled Electrical Drive and Method of OperatingSuch Drive," Ser. No. 583,146, filed Sept. 9, 1966 now US. Pat. No.3,470,434 is also hereby incorporated by reference into this applicationfor the purpose of fully describing a suitable silicon control rectifieramplifier.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to motor control systems and has particular relation to thevariable-voltage motor drive systems utilized in elevator systems.

2. Prior Art The concurrently filed application of William R. Caputoentitled Motor Control System and incorporated by reference into thisapplication, describes a variable-voltage elevator drive systemutilizing a silicon-controlled rectifier amplifier to excite the fieldof the direct current generator directly from an alternating currentsource. In that system, a speed reference signal generated as acontinuous function of the position of the car relative to the landingsis compared with the actual speed of the motor in a drag magnetregulator. The error signal thus produced is combined with a bias signalwhich compensates for the lag in the response of the elevator car. Theresultant signal controls the gating of portions of half-cycles of analternating current source by the SCR amplifier to the generator fieldcircuit. The bias signal is also necessary to initiate movement of thecar since the pattern generator will generate zero voltage when the caris stopped level with a landing. Should the car become displaced fromthe landing through stretch or contraction of the cable by a shift inthe loading of the car, releveling is accomplished by the error signalgenerated by the position-responsive pattern signal generator. Since thepattern signal is generated as a function of the displacement of a carfrom a landing, precautions must be taken to avoid excessive jerk shouldthe car he started from a point in-between the floors such as after anemergency stop. Therefore, if the car starts from outside of theleveling zone, a positive limitation is imposed on the speed attainableby limiting the potential of the alternating current supplied to the SCRamplifier until resynchronization is accomplished at the next stop. Highloop current flowing a predetermined time after the brake is set shutsdown the system. Various feedback circuits such as a stabilizationfeedback circuit and a residual killer circuit are provided. A feedbacksignal proportional to the back electromotive force of the motor is alsosupplied when the system is operating on hand control when the patterngenerator is not being utilized.

US Pat. No. 3,448,364 discloses a scheme for bringing an elevator car toa stop should a speed control system which regulates the speed of thecar during deceleration as a function of the remaining distance to gofail to initiate deceleration by a predetermined point as the carapproaches a terminal landing. However, in that system power is cut offand the brake is set so that the car is brought to rest without anyreference to the terminal landing. This leaves the passengerstemporarily stranded in the car with the doors closed.

SUMMARY OF THE INVENTION It is a primary object of the invention toprovide an improved variable-voltage motor drive system.

It is an important object of the invention to provide an improvedvariable-voltage motor drive system of the type in which a high-gainamplifier controls the field excitation of a direct current generator asa function of the difference between the desired speed of the motor asrepresented by a signal generated by a pattern generator and the actualspeed of the motor as represented by a signal generated by a tachometergenerator wherein fail-safe means are provided to bring the car to asmooth stop should either the pattern generator or the tachometergenerator fail as an open circuit.

It is a more particular object of the invention to provide avariable-voltage elevator drive system as described in the previousobject wherein the fail-safe means is switchless and compriseshigh-impedance circuits in parallel with the pattern generator and thetachometer generator which are normally short circuited by the signalgenerator which they shunt.

It is an even more particular object of the invention to provide avariable-voltage elevator drive system as described in the previousobject wherein the high-impedance circuit in parallel with thetachometer generator includes the armature circuit of the motor so thatthe pattern signal can be overridden and the car can be brought to aslow speed should the tachometer generator fail as an open circuit.

It is also an object of the invention to provide means to limit the jerkin a variable-voltage drive system used as an elevator drive whenswitching from acceleration to deceleration on short floor runs.

It is a further object of the invention to provide a variablevoltageelevator drive system as described in the previous object which does notinterfere with leveling.

It is another object of the invention to provide means to bring anelevator car into a terminal landing at a safe speed should a patterngenerator, which generates a speed reference signal as a function of thedistance from a landing at which the car is to stop, malfunction andfail to initiate deceleration by a predetermined point as the elevatorcar approaches the terminal landing.

It is yet another object of the invention to provide means to preventpreopening of the doors when an elevator car is being driven under slowspeed conditions.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram of a portion of anelevator system incorporating the invention; and

FIG. 2 is a circuit diagram in straight line form of a portion of asupervisory system which controls the operation of the elevator systemdisclosed in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION The preferredembodiment of the invention will be shown as applied to the basic systemdisclosed in the concurrently filed application Ser. No. 837,442 ofWilliam R. Caputo. Portions of the system not shown in detail in thisapplication can be considered to be the same as shown in the otherapplication. Where portions of the basic circuit must be shown in detailfor an understanding of the present invention the reference charactershave been preserved to facilitate cross referencing.

As an aid to understanding the drawings the relays an switches areidentified as follows:

A Brake monitor relay.

AE Landing zone relay.

B Holding relay.

CL Current limiter relay.

CLT Current limiter delay relay.

DSD Down slowdown switch.

L Pattern leveling zone relay.

LOT Left full advance switch.

PHL Photoelectric relay.

ROT Right full advance switch.

S Selector relay.

TSD Terminal slowdown relay.

USD Up slowdown switch.

VT Stopping timer relay.

YA Auxiliary loop current monitor relay.

Z1 Leveling zone switch.

Z2 Door preopening switch.

1 Up direction relay.

2 Down direction relay.

38 Running relay.

22 Door preopen relay.

29 Safety circuit relay.

34X Auxiliary stop signal.

40 Car door signal.

55 Overspeed relay.

60 Automanual relay.

The relay contacts are identified by hyphenated reference characters.The portion of the reference character before the hyphen identifies therelay with which the contacts are associated and the number after thehyphen identifies the particular contacts on the associated relay. Allof the relay contacts are shown in their normal position when the relayis deenergized. For instance, the make contacts CL-5 in FIG. 1 are openwhen the relay CL is deenergized and closed to complete an electricalcircuit when the relay 1 is energized. On the other hand, the brakecontacts CL-6 are closed when the relay CL is deenergized and are openedwhen the relay CL picks up.

Referring to FIG. 1, a traction sheave 5 is connected to the shaft of adirect current motor 1. An elevator car identified by the generalreference character 2 is supported by a cable 7 which is reeved over thetraction sheave 5. A counterweight 11 is fastened to the other end ofthe cable 7. Rotation of the traction sheave 5 by the motor 1 causes theelevator car 9 to ascend and descend relative to landings including alower terminal landing, an upper terminal landing and a number ofintermediate landings.

The elevator car 9 is provided with a door 61 the operation of which iscontrolled by a door operator 63 mounted on the top of the car. Thelanding zone detector 65 mounted on the car comprises a light source anda photoelectric cell connected to a relay PHL (not shown) which isenergized as long as the light from the light source impinges upon thephotoelectric cell. When the elevator car reaches a point level with alanding, a plate 67 mounted in the hatchway adjacent each landinginterrupts the light beam thereby deenergizing the relay PI-IL. Thedimensions of the plate 67 and the landing detector 65 are such that therelay Pl-IL will only be deenergized when the car is withinapproximately plus or minus one-fourth of an inch from being level withthe associated landing.

A cam 69 mounted on the car serves to operate a number of normallyclosed switches as the elevator car moves up and down in the hatchway.As the car approaches the upper ter' minal landing as shown in FIG. 1the cam 69 operates the up slowdown switch USD. When the car reaches apoint just slightly above the point where the floor of the car is levelwith the upper terminal landing, the cam 69 operates the up limit switchUL. Similarly, when the car approaches the lower terminal landing thecam 69 operates the down slowdown switch DSD and-upon continued downwardmovement operates the down limit switch DL when the car reaches a pointslightly below the point where the floor of the car is level with thelower terminal.

The direction and speed of rotation of the motor 1 is controlled by theoutput of the direct current generator 3. A loop circuit formed by theinterconnection of the armatures of the generator 3 and the motor 1 areshown in heavy lines in FIG. 1. The armature of the generator 3 isdriven at a constant r.p.m. by an alternating current motor (not shown).The output of the generator 3 is controlled by the variable energizationof the generator field winding F.

The field winding F of the generator is excited by thesilicon-controlled rectifier amplifier 17. A detailed description of theSCR amplifier is given in the Caputo application Ser. No. 583,146, filedSept. 9, 1966 which has been incorporated by reference into thisapplication.

For present purposes a description of the operating characteristics ofthe SCR amplifier is sufficient. The device receives alternating currentfrom a 1l0-volt alternating current supply through a transformer T3which has a center-tapped secondary. Portions of alternate half-cyclesof the two opposing alternating current voltages thus produced are gatedby the SCR amplifier to the field of the generator either through chokeCH1 or CH2 depending upon the polarity of the control signal applied tothe input terminals c-d of the SCR amplifier. The amplitude of thehalf-cycles of the alternating current voltage delivered to the SCRamplifier is unaffected by the amplifier; however, the instant duringthe half-cycle when the selected half-cycles are gated to the generatorfield is a function of the magnitude of the input signal.

The amplitude of the half-cycles gated to the generator field isaffected by the current limiter. With the car running under normalconditions, make contacts CL-S of the current limiter relay, theoperation of which will be explained below, are closed while the breakcontacts CL-6 are open. Under these conditions the full llO-volt supplyvoltage is applied across the primary of T3. However, with the currentlimiter relay deenergized, make contacts CL-S are open and breakcontacts CL-6 are closed. This interposes transformer TR2 between thesupply and the primary of T3. Since transformer TR2 is a stepdowntransformer the amplitude of the half-cycles gated to the generatorfield is reduced. The capacitor C5 and resistor R24 are placed in seriesacross the primary of T3 to smooth out the interjection and removal ofTR2 in the circuit.

Several signals, singly and in combination, can be applied to the inputterminals c-d of the SCR amplifier. One such signal is generated by thepattern potentiometer PP. This potentiometer has a fixed center tap 71and a slider 73. The pattern potentiometer receives energization form afull-wave rectifier 75 which in turn is energized from a 1l0-volt ACsource through the make contacts 5-] of the selector relay. Directcurrent output voltages of opposite polarity with respect to the neutraloutput lead g appear at the outputs e and f of the fullwave rectifier75. The neutral output g is connected to the center tap 71 of thepattern potentiometer. The output e is connected through variableresistor R27 to one end of the pattern potentiometer PP while the outputf is connected through variable resistor R28 to the opposite end of thepattern potentiometer.

Displacement of the slider 73 from the point opposite the fixed tap 71introduces a voltage into the circuit connected to the input terminalsc-d of the SCR amplifier. The direction of the displacement determinesthe polarity of the voltage introduced and the amount of displacementdetermines the amplitude of the signal. The amplitude of the signal fora given displacement can be trimmed by the variable resistor R27 or R28depending upon the direction of displacement. Of course these variableresistors also set the maximum signal that can be generated for maximumdisplacement of the slider.

Displacement of the slider in one direction from the neutral positionwill produce an input voltage for the SCR amplifier of a polarity whichwill cause the motor to turn in a direction which will raise theelevator car. Displacement of the slider in the opposite direction willproduce a voltage of the opposite polarity and cause the car to descendin the hoistway. When the slider 73 is directly opposite the center tap7], zero voltage is introduced into the SCR amplifier input circuit.

The slider 73 is connected to the pattern generator 22. The patterngenerator in turn is connected to the elevator car 9. Upon command fromthe supervisory control 28, the pattern generator 22 displaces theslider 73 on the pattern potentiometer from the neutral position in adirection and at a rate corresponding to the desired direction ofmovement and rate of acceleration of the elevator car. The magnitude ofthe displacement determines the speed of the car. As the car approachesa floor at which it is to stop, the pattern generator will return theslider to the neutral position at a predetermined rate corresponding tothe desired deceleration. A suitable pattern generator adapted for usewith this system is disclosed in the Savage application mentioned aboveand incorporated by reference into this application. The Savage patterngenerator controls acceleration as a function of the distance the carhas traveled until a predetermined maximum speed is reached and controlsdeceleration as a function of the distance remaining to a floor at whichthe car is to stop.

The voltage produced by the pattern potentiometer PP is supplied to theinputs cd of the SCR amplifier through the resistor R29. The patternpotentiometer PP is shunted by the capacitor C6. This capacitor C6 andthe effective resistance introduced into the input circuit of the SCRamplifier by the pattern potentiometer PP, serve as an RC filter todampen changes in the pattern signal. Since the effective resistance ofthe pattern potentiometer is higher for the higher speed settings of theslider 73, it can be seen that the filter automatically increases theamount of damping as the speed increases. It can also be appreciatedthat the damping is minimal at the slow-speed settings so that theresponse of the system during leveling is unaffected. The effect of thefilter is especially use ful on one floor runs when the car is switchedfrom acceleration to deceleration before acceleration has been fullycompleted. The sudden switch in the mode of operation can causediscomfort to the passengers and unnecessary strain on the equipment.The filter, however, smoothes the transition from acceleration todeceleration yet it does not cause the car to overshoot since thedamping is gradually removed as the car approaches the landing at whichit is to stop. It can be seen then that the capacitor C6 and patternpotentiometer PP form a variably damped signal generator.

Since the pattern generator develops the speed reference signal as afunction for the distance the car has traveled from the floor at whichit started, difficulties are encountered when a car is brought to a stopbetween floors as when the emergency stop button is pulled. As was fullyexplained in the Caputo application, assignees Case No. 38,779 mentionedabove, a severe jolt would be introduced into the system if this signalwas suddenly reapplied when the car was restarted. Under thesecircumstances therefore, the current limiter relay CL is energized in amanner to be disclosed below. As was already seen, the deenergization ofthe relay CL reduces the amplitude of the alternating current voltagesupplied to the SCR amplifier. In addition, when the system is operatingunder automatic control so that the contacts 60-13 are closed, thecontacts CL-7 close upon deenergization of the current limiter relay tointroduce the resistor R30 in parallel with the pattern potentiometer.Introduction of the resistor R30 into the control circuit has the effectof further reducing the magnitude of the pattern signal applied to theinputs cd ofthe SCR amplifier.

Another signal which serves as an input to the SCR amplifier is suppliedby the tachometer generator 77. The tachometer generator is mechanicallydriven by the motor 1 and produces an output voltage proportional to thespeed of rotation of the motor ll. The tachometer generator is soconnected in the input circuit of the SCR amplifier that it opposes thepattern signal being developed by the pattern potentiometer PP. In otherwords, when the pattern potentiometer is producing a signal of a givenpolarity to cause the motor M to rotate in a certain direction, therotation of the motor in this direction will cause the tachometergenerator to generate a voltage opposite in polarity to the polarity ofthe signal being generated by the pattern potentiometer. These signalsare combined in the input circuit of the SCR amplifier and the errorsignal thus developed controls the operation of the SCR amplifier.

Several other signals can be included in the error signal which controlsthe SCR amplifier. For instance the bias circuit 37, which was fullydescribed in the concurrently filed Caputo application, introduces asignal into the input of the SCR amplifier by applying a voltage acrossthe resistor R25. The bias circuit receives its energization from afull-wave rectifier 79 through either the 1-6 make contacts of the updirection relay or the 2-6 make contacts of the down direction relay,when the car is on automatic operation, make contacts 60-10 of theautomanual relay are closed, the supervisory system is not generating astop signal, make contacts 34X-2 of the auxiliary stopping relay areclosed, and the selector relay is energized, contacts 8-2 are closed. Asis explained fully in the Caputo application, the bias signal isnecessary in order to initiate movement of the car and to compensate fora lag in the response of the elevator car to the pattern signal. Thebias circuit contains a filter circuit so that the signal generated isapplied and removed smoothly.

Alternatively, a signal may be applied across the resistor R25 when thecar is on automatic operation through the resistor R26 when the breakcontacts TSD-1 of the terminal slowdown relay are closed. The polarityof the signal thus applied depends upon whether the contacts 1-6 or 2-6of the up or down direction relays are closed. As will be seen below,when the contacts TSD-l are closed the contacts S-1 and 5-2 of theselector relay are open to remove energization from the patternpotentiometer and the bias circuit. Under these circumstances, the fixedsignal applied through the resistor R26 will generate a pattern signalwhich will cause the car to continue to travel in the direction in whichit was traveling but at a greatly reduced speed, on the order of 30 feetper minute.

As was also fully explained in the concurrently filed application ofCaputo, at times it is desirable to position the car at points in thehatchway intermediate the landings for maintenance purposes. To effectsuch control the break contacts 60-11 of the automanual relay are closedto supply current from the full-wave rectifier 79 to the hand controlpattern generator 41 through either the up direction or down directionrelay contacts 1-6 or 2-6. The hand control circuit will therefore applya voltage across the resistor R25 which will cause the car to travel inthe selected direction at a speed of approximately 60 feet per minute.

The signal generator formed by the pattern potentiometer PP and theresistor R25 is shunted by resistor R31. Since the resistors R25 and R29are typically on the order of 1.5 K. and the resistance of the patternpotentiometer PP varies typically from a value of 0 to 1 K., theresistor R31 which is on the order of K. is ineffective under normalconditions. However, should the pattern potentiometer fail as an opencircuit the resistor R31 automatically, and without switching, completesthe circuit for the application of the voltage of the tachometergenerator 77 to the inputs of the SCR amplifier. Due to the large valueof resistance of the resistor R31, the voltage output of the tachometergenerator is greatly attenuated but since it is opposing the directionof rotation of the motor 1 it will bring the car smoothly to a creepspeed.

It will be noticed that the resistor R36 and the armatures of the motor1 and generator 3 form a shunt circuit around the tachometer generator77. Since the effective resistance of the tachometer generator is on theorder of 60 ohms and the resistor R36 typically has a value of 660 K.,this feedback circuit is ineffective under normal conditions. However,should the tachometer generator 77 fail as an open circuit, the voltageacross the motor armature would be introduced automatically and withoutswitching into the input circuit of the SCR amplifier through theresistor R36. Since the voltage across the motor armature at designspeed is on the order of 220 volts, the feedback signal applied throughthe resistor R36 which opposes the signal being developed by the patterngenerator and the bias circuit would exceed the pattern signal tendingto cause the car to travel in a given direction and again bring the carsmoothly to a creep speed. Of course, as the speed of the motor isslowed down, the feedback signal decreases until an equilibrium betweenthe pattern signal and the feedback signal is reached.

Several additional feedback circuits are provided between the armatureloop circuit and the input of the SCR amplifier. A stabilization circuitfeeds back a negative signal which opposes changes in speed throughresistor R32, capacitor C8 and resistor R33. A residual killer circuitprovides another feedback signal through the resistor R32 and the breakcontacts B2 of the holding relay. This latter circuit is renderedinoperative during car movement by the energization of the B relay.However, after the car has stopped and the brake is set, the relay B isdeenergized to close the contacts B2 so that a signal proportional tothe loop current is fed into the SCR amplifier with a polarity tendingto drive the loop current to zero. The closing of the contacts B2 alsoserves to complete a discharge path for the capacitor C8 through theresistor R33. The operation of the stabilizing and residual killercircuits is fully discussed in the concurrently filed Caputoapplication.

An additional negative feedback circuit is provided through the resistorR34 the back to back Zener diodes ZDI and ZD2, and a capacitor C7. Ifthe direction of rotation of the motor is such that the anode of theZener diode ZD2 is positive with respect to the input of the SCRamplifier, ZD2 will be forward biased, however, flow of current in thefeedback circuit will be blocked by the Zener diode ZDI until thevoltage across this diode exceeds its Zener voltage. When this voltageis exceeded, a further increase in the armature voltage will causecurrent to be passed through the two Zener diodes and through thecapacitor C7 to the input c of the SCR amplifier. This circuit providesadditional damping at the higher speeds when the voltage across themotor armature exceeds the Zener breakdown voltage of diode ZDI or ZD2.Since the motor armature voltage is approximately 220 volts at designspeed, utilization of l lO-volt Zener diodes f or ZDl and ZD2 providesfor the additional damping above half speed. Again, this is particularlyuseful on one floor runs where the car has not quite completedacceleration before the pattern switches to deceleration. When thevoltage across the motor armature begins to decrease, the Zener diodeZDl becomes forward biased however, the voltage differential across ZD2will be below the Zener voltage. Under these circumstances the capacitorC7 will discharge through the resistor R35'to apply a voltage across theinputs c-d of the SCR amplifier. The resistor R35 is very large,typically on the order of I50 K, to provide slow discharge of thecapacitor C7.

When the motor M is turning in such a direction that the anode of theZener diode ZD2 is negative with respect to the input 0 of the SCRamplifier, the Zener diode ZDI is forward biased but the diode ZD2 willblock the flow of current in the feedback circuit until the voltageacross the motor armature exceeds the Zener breakdown voltage of ZD2.Above this voltage a feedback circuit will be supplied to the input ofthe SCR amplifier through the series RC circuit composed of capacitor C7and resistor R34. As the magnitude of the negative potential across themotor armature decreases, the Zener diode ZDI becomes reverse biasedand, since due to the stored charge on the capacitor C7 the bias acrossZDl does not exceed its Zener breakdown voltage, the capacitor C7 willdischarge through the resistor R35 and the input of the SCR amplifier tooppose the reduction in speed.

Turning to FIG. 2, the safety circuit relay 29 is energized by thebusses L1 and L2 when all the contacts of the safety circuits 81 areclosed. The safety circuits have been widely used for many years in theelevator industry and therefore need not be described in detail. It isenough to say that the safety circuits conventionally include contactsof the governor which trips when the speed of the car exceeds a desiredspeed by a predetermined amount, the emergency stop button, up and downover travel switches and perhaps such other switches as the emergencyhatch open switch which indicates that the safety hatch at the top ofthe car is not in the closed position. It

can be appreciated therefore that under normal operating conditions thecircuit is completed through the safety circuits 81.

The up direction relay 1 is energized when the safety circuits areclosed and the up limit switch UL is closed if the direction circuitshave indicated that the car should travel in the up direction. Allelevator circuit systems have direction circuits in one form or anotherto indicate the direction in which the car should travel. Since thedirection circuits to be used herein are identical to those disclosed inthe concurrently filed Caputo application they are represented in FIG. 2by a block labeled 83. The circuit is completed for energization of thedirection relays either through the output of the direction circuitslabeled n which is connected directly to the bus L2 or through the moutput which is connected to the bus L2 either through the make contacts35-1 of the running relay or the make contacts 55-1 of the overspeedrelay. It will be seen from reference to the circuits disclosed in theconcurrently filed Caputo application that the contacts 55-I of theoverspeed relay must be closed in order for the circuit to be completedinitially to energize the l relay. Once the relay 1 is energized, itscontacts 1-4 close to energize the relay 35. With the relay 3S energizedthe relay 1 is held in through the 38-1 contacts. Similarly the downdirection relay 2 is initially energized through the switch DL and inturn energizes the running relay 38 through the contacts 2-4. It is onlyduring the leveling phase on automatic control that the directioncircuits are energized through the n output. It can be seen from FIG. 2that if the car ascends or descends to the point where the up limitswitch UL or the down limit switch DL respectively is opened by the camon the car that the relays 1 or 2 and 35 are deenergized.

The leveling zone relay L is energized when the safety circuits areclosed, make contacts 29-1 are closed, and the car is on automaticoperation, contacts 60-7 are closed, when the car is within plus orminus 2 inches from being level with the floor at which the car ismaking a stop, switch Z1 closed. As mentioned above, it is assumed thatthe Savage floor selector which has been incorporated by reference intothis application is being utilized. In the Savage floor selector thecontrol head is rotated either clockwise or counterclockwise from avertical position as the car is accelerated. Deceleration is controlledby the return of the control head to the neutral vertical position. Asthe control head approaches the neutral vertical position a cam 215 inthe Savage application on the control head operates a push switch 221 asthe car reaches a point 2 inches from the landing. The switch Z1 in FIG.2 corresponds to the switch 221 in the Savage disclosure.

The current limiter switch CL can also only be energized when the car ison automatic control and the safety circuits are closed. The relay CLcan only be energized initially if the car is started in the levelingzone, switch Z1 closed and break contacts S-3 of the selector relay areclosed. In addition, the overspeed relay 55 must be energized indicatingthat the car has not been in an overspeed condition, contacts 55-2closed. Furthermore the doors of the car must be closed so that thecontacts CLT-1 of the current limiter timer relay are closed. ()nce therelay CL is energized it is held in by its holding contact CL-% and themake contacts 13-1 of the holding relay once the car is running.

The current limited timer relay also will only be energized when thesafety circuits are closed and the car is on automatic control. Inaddition the car door must be closed, make contacts 40-2 closed. Therelay CLT is provided with a 2-second delay in dropout. Devices forachieving this delay in dropout such as a series RC circuit shunting therelay are well known in the elevator art.

The landing zone relay AE will be energized when the safety circuits areclosed and the car is on automatic control as long as the make contactsPI-IL-ll of the photoelectric relay are closed. It will be rememberedthat this relay is energized continuously except when the car is withinplus or minus onefourth of an inch of being level with the landing. Therelay AE is provided with a l-second delay in dropout to preventdeenergization of this relay as the car passes floors at which it is notto stop.

The stopping timer relay VT is energized when the safety circuits areclosed and the car is on automatic control when the running relay isenergized, contacts 35-3 closed and the brake has been lifted, contactsA-fi of the brake monitor relay are closed. As was discussed in theconcurrently filed Caputo application the standard elevator brake isspring biased to the braking condition. The brake is released by anelectromagnet when the car is to start. The relay A is energized whenthe brake plunder has moved a short distance indicating that the brakehas been released. The relay VT is also provided with a -second relay indropout.

The terminal slowdown relay TSD can only be picked up through the breakcontacts YA-3 of the auxiliary loop current monitor relay. As was fullydiscussed in the concurrently filed Caputo application, this relay ispicked up and remains picked up once the motor generator set is put onthe line. Therefore the relay TSD is energized-when the power isinitially applied to the system. Once picked up, the relay TSD remainsenergized through its holding contacts TSD-2, either the down slowdownswitch DSD or the right full advance switch ROT, and either the upslowdown switch USD or the left full advance switch LOT. The switchesLOT and ROT correspond to the switches 227 and 229 in the Savageapplication which are opened when the control head on the floor selectorreaches the full advance position and the car is traveling at full speedin the up or down direction respectively.

As was mentioned previously, the normally closed switch USD is openedmomentarily by a cam on the car as the car ap proaches the upperterminal. Similarly the normally closed switch DSD is opened momentarilyby the cam on the car as the car approaches the lower terminal. It canbe seen that if the car has not begun to decelerate so that the switchLOT is still open as the car approaches the upper terminal and theswitch USD is opened, the terminal slowdown relay TSD will be droppedout. Likewise, if deceleration has not been initiated as the car reachesa predetermined point in the hatchway as it approaches the lowerterminal, both the' switches ROT and DSD will be opened to drop out therelay TSD. Once the relay TSD is dropped out it cannot be reenergizeduntil a maintenance man recycles the MG switch.

The overspeed relay 55 can only be energized if the terminal slowdownrelay is picked up (contacts TSD-3 are closed). This relay remainsenergized as long as the switch OS remains closed. The normally closedswitch OS on the governor will open when the speed of the car exceedsthe design speed by 10 percent. Once tripped the switch OS remains openuntil it is reset.

A selector switch S in energized while the car is running on automaticcontrol, make contacts FT-l of the stopping timer relay are closed, aslong as the safety circuits are closed and the terminal slowdown relayis picked up, contacts 29-2 and TSD-4i are closed.

The holding relay B is energized whenever either the running relay orthe selector relay are energized, contacts 38-6 or 5-4 are closed.

It is conventional in modern elevator systems to initiate opening of thecar doors as the car approaches the landing to expedite the transfer ofpassengers. However, when the car is being run at slow speed, whichunder the system described occurs when the relay CL is dropped out, thedoor should not be opened before the car reaches the floor sincepassengers will be inclined to trip when the doors open before the caris level. The door preopening relay 22 will be energized to initiatepreopening of the doors when a stop signal has been generated, breakcontacts 34X-4 of the auxiliary stopping relay are closed, as the carapproaches the landing, switch Z2 closed, if the car is operating undernormal conditions, contact CL-9 of the current limiter relay are closed.The switch 22 corresponds to the switch 223 on the Savage floor selectorwhich is closed by the cam 217 when the car approaches within 10 inchesof the landing at which it is to stop. The door control circuits arewell known in the art. The energization of the relay 22 can be utilizedto generate a signal 22 which will initiate door opening through thecircuits of the supervisory system disclosed in the Kirsch applicationmentioned above. The relay 22 will also be energized when a car which isidle at a floor, switch Z2 and contacts AE-2 are closed, if the car isassigned to a corridor call which is registered at that floor, contacts34X-4 close.

OPERATIONS As an aid to understanding the invention, some typicaloperations should be considered. Assume that the elevator car is idle atthe lower terminal floor with the doors open. Under these conditions thedirection relays I and 2 and the running relay 38 will be deenergized.With the relay 3S dropped out, the relay VT and therefore the selectorrelay S will be deenergized. With the contacts S-ll and 5-2 in FIG. 1open no power will be supplied to the pattern potentiometer or the biascircuit. With the contacts 38-2 in FIG. 2 open, the relay CL will bedropped out so that the contact CL-S in FIG. 1 will be open while thecontact Cir-6 will be closed. Under these circumstances then, reducedpotential will be supplied to the SCR amplifier 17 however, since noinput signal is being supplied to the amplifier no portion of thehalf-cycles of the alternating current potential applied to the SCRamplifier will be delivered to the field ofthe generator. With the dooropen, the contacts 4tl-2 will be open so that the relay CLT will bedeenergized. The relay TSD will be energized at this point through itsholding contacts TSD-2 while the contacts YA-3 are open.

Assume now that a passenger enters the elevator car at the lowerterminal and registers a call for the second floor. Conventionalsupervisory circuits (not shown) will complete circuits to close the cardoors. When the car doors are closed the contacts 40-2 will close toenergize the relay CLT which has no effect on the system at this point.Circuits within the supervisory system will also activate relays whichwill complete the direction circuits through the closed overspeedcontacts 55-1 to energize the up direction relay 1. Energization of thisrelay results in closing of the contacts 11-4 to energize the runningrelay 38. Contacts 3S-l of the running relay in parallel with theoverspeed relay contacts will therefore be closed. Since the car iswithin the first floor leveling zone, the switch Z1 will be closed andthe relay CL will be energized through the contacts 5-3 of the selectorrelay, the contacts 55-2 of the overspeed relay and the now closedcontacts 35-2 of the running relay.

Energization of the CL relay results in the application of fullpotential to the SCR amplifier through the closure of the CL-S contactsand the opening the CL-6 contacts in the power supply circuit. Openingof the CLI-7 contacts disables the resistor R30 shunting the patternpotentiometer PP.

Energization of the 3S relay results in energization of the brake relay(not shown) and when the brake plunger is released the brake monitoringrelay A (not shown) is energized. Closure of the A-Z contacts in FIG. 2completes the circuit for energization of the stopping timer relay VT.Closure of the contacts VT-ll completes a circuit for energization ofthe selector relay S. Opening of the 8-3 contacts does not result indropout of the CL relay however since the relay is held in by the CLT-llcontacts. Closure of the 8-11 contacts in FIG. I completes the circuitfor energization of the full-wave rectifier of the pattern signalgenerator however, no signal is supplied by the pattern potentiometer PPto the SCR amplifier at this time since the movable tap 73 is oppositethe fixed tap 71 when the car is at rest at a landing. Since thecontacts 2-6 of the up direction relay, contacts 60-10 of the automanualswitch and the contacts EMX-Z of the auxiliary stopping relay are allclosed at this time, the bias circuit is energized upon the closure ofthe 8-2 contacts. It will be remembered that at this point the terminalslowdown relay TSD is energized so that the contacts TSD-1 are open.Energization of the bias circuit 37 results in the application of avoltage across the resistor R25 which introduces a signal into theinputs of the SCR amplifier tending to energize the motor to cause thecar to travel in the up direction.

As the car begins to move, a voltage tending to oppose the voltageintroduced by the bias circuit is generated by the tachometer generator.Since this signal will be less than the signal introduced by the biascircuit, the resultant error signal will continue to cause the car toaccelerate in the up direction. Movement of the car also results inoperation of the pattern generator 22. The pattern generator begins todisplace the slider 73 on the pattern potentiometer PP in a directionwhich produces a signal tending to increase the speed of the car in theup direction. Since the resistance introduced into the input circuit ofthe SCR amplifier at this point is very small the damping effect causedby the pattern potentiometer in the capacitor C6 is very small at thispoint. The increase in the input signal to the SCR amplifier causes themotor to turn faster which in turn causes the car to advance farther inthe up direction. The pattern generator in turn displaces slider 73farther from the neutral position as a function of the distance the carhas traveled. The car will therefore quickly and smoothly accelerate inthe upward direction. When the speed of the car exceeds half speed thevoltage across the motor ar' mature will exceed the Zener breakdownvoltage of say the Zener diode ZDl. The feedback circuit thereforeapplied to the SCR amplifier through the resistor RM and capacitor C7will tend to resist further increase in the speed of the car andtherefore will tend to dampen changes in the car speed.

As the car is accelerated in the upward direction the cam 69 (seeFIG. 1) will momentarily open the switch DSD. However, since the carwill still be accelerating so that the right full advance switch ROT onthe pattern generator will remain closed, the relay TSD will remainenergized.

Assuming that the car requires more than half the distance between thefirst and second floor to accelerate to full speed, the stop signal forthe second floor will be generated before the car has completedacceleration. Generation of the stopping signal results in the openingof the 34X-2 contacts of the auxiliary stopping relay to deenergize thebias circuit. The bias signal will not be removed instantaneouslyhowever due to the energy stored in the capacitors in the bias circuit.Generation of the stopping signal also causes the pattern generator tobegin to return the movable tap 73 towards the neutral position oppositethe fixed tap 71. Since an appreciable amount of resistance is insertedin parallel with the capacitor C6 by the pattern potentiometer at thispoint the charge stored on the capacitor C6 tends to oppose andtherefore dampen any sudden effect of the reduction in the patternsignal.

Furthermore as the motor begins to slow down due to the reduction in thepattern potential, the charge stored on the capacitor C7 reverse biasesthe diode ZD2. This charge stored on a capacitor C7 also opposes anysudden reduction to the input signal applied to the SCR amplifier. Thischarge will be dissipated slowly through the large resistor R35 and theinput circuit of the SCR amplifier.

As the pattern potentiometer is brought to the neutral position by thepattern generator, the switch Z2 on the control head of the patterngenerator will be closed when the car is approximately lO inches belowthe level of the second floor. Since at this point the auxiliarystopping relay 34X is deenergized so that the contacts 34X-4 are closedand since the contacts CL-9 of the current limiting relay are closed thedoor preopening relay 22 will be energized. The energization of thisrelay will initiate door opening through conventional door controlcircuits (not shown). When the car reaches the level position the plate67 will interrupt the photoelectric circuit in the photoelectricdetector 65 to drop out the relay PHL and therefore open the contactsPHL-l in FIG. 2. One second later the landing zone relay AE will dropout to interrupt the circuit for the up direction relay 1 and therunning relay 38 in their direction circuits. One-half second after therelay 35 is deenergized the relay VT will drop out to deenergize the Srelay. Deenergization of the S relay will remove the power supplies fromthe full-wave pattern potentiometer and also open the contacts S-2through the bias circuit. With the contacts 35-6 and now 8-4 open therelay B will be deenergized to close the contacts 13-2 in the residualkiller circuit shown in FIG. 1. Completion of this feedback circuit willresult in the application of a signal to the SCR amplifier tending todrive the loop current in the motor and generator armature circuits togo to zero.

Assume now that the car is assigned to proceed to the top terminallanding. The doors will close and the car will accelerate in the updirection in the manner previously described. Assuming that there areseveral floors between the second landing and the upper terminal, thecar will reach full speed when the control head on the pattern generatorreaches the full advance position and opens the switch ROT. Assume nowthat a malfunction occurs in the pattern generator as for example assumethat the drive tape becomes disconnected so that the control headremains in the full advance position. Under these conditions movement ofthe car will not be detected and no stop signal will be generated whenthe car approaches the upper terminal.

When the car passes the point where deceleration for a stop at the upperterminal would normally be begun, cam 69 on the car will momentarilyopen the switch USD. Normally this would have no effect on the systembecause if deceleration had been initiated the control head would not bein the full advance position and the switch LOT would be closed tomaintain the terminal slowdown relay TSD energized. However, with thepattern generator locked in the full advance position so that the switchLOT remains open, the momentary opening of the switch USD results indropout of the terminal slowdown relay TSD. Deenergization of this relayopens the holding contacts TSD-2 so that the relay is not reenergizedwhen the switch USD again closes as the car continues up the hatchway.As was mentioned previously, the contacts UA-3 of the auxiliary loopcurrent monitor relay remain open once the motor generator set has beenplaced on the line. Therefore, the terminal slowdown relay TSD remainsdeenergized.

The dropout of the relay TSD causes the contact TSD-4 to open therebydeenergizing the selector relay S. Opening of the contacts S-1 and 5-2results in deenergization of the pattern potentiometer and the biascircuit respectively thereby removing the signals tending to cause thecar to travel in the up direction. However, closure of the contactsTSD-1 results in the application of a voltage across the resistor R25tending to cause the car to continue traveling in the up direction sincethe contacts l6 remain closed. The sudden reduction in the up directioncommand signal is dampened somewhat by the damping effect of thecapacitors C6 and the large resistance introduced in the circuit by thepattern potentiometer at this point. The sudden change in the speed ofthe system is also opposed by the charge stored on the capacitor C7.

Opening of the contacts TSD-3 results in dropout of the overspeed relay55. Opening of the contacts 55-2 deenergizes the relay CL which in turnreduces the potential the alternating current applied to the SCRamplifier through the transformer T3. The car will therefore proceedupward toward the upper terminal at a fixed speed, typically at about 30feet per minute.

Under the conditions described, that is where the control head on thepattern selector becomes stranded in the full advance position, theswitch 22 will not be closed to initiate preopening of the doors as thecar approaches the landing. Opening of the contacts 55-1 does not resultin deenergization of the up direction relay and the running relay 38since these relays are held in by the contacts 3Sl of the running relay.When the car reaches a point slightly above the normal level positioncam 69 on the car will open the up limit switch UL to deenergize thedirection relay 1 and the running relay 38. Opening of the contacts 1-6results in the removal of the signal tending to drive the car in theupward direction and dropout of the relay 35 results in application ofthe brake (not shown).

By referring to the concurrently filed application of Caputo it will beseen that the running relay 3S and the direction relay cannot bereenergized until the contacts 55-1 of the overspeed relay are reclosed.However, it can be seen from FIG. 2 that the relay 55 cannot bereenergized until the contacts TSD-3 of the terminal slowdown relay arereclosed. This relay cannot be reenergized, it will be remembered, untilthe motor generator set is shut down and restarted. Therefore, the carwill remain at the upper terminal until a maintenance man corrects themalfunction.

It should be noticed that should the overspeed relay be deenergizedbecause the car exceeds the design speed by over percent, that again thepotential to the SCR amplifier will be reduced through thedeenergization of the relay CL. The car will continue traveling underthe control of the pattern generator through manipulation of the patternpotentiometer, however, the car will travel at a lower speed. Thepattern signal will be further reduced by the shunt resistor R30 throughthe closure of the contact CL7. Since the car is traveling at a muchlower speed it would not be desirable to open the doors when the carreached a point 10 inches from the landing in order to protect peoplefrom stumbling over the landing sill. Consequently as the car reaches apoint 10 inches from the landing and the pattern generator closes theswitch 22 the contact CL-9 will now be opened so that the doorpreopening relay 22 cannot be energized. Since the car is not yet levelwith the landing the contacts AE-2 of the landing zone relay will alsobe open.

Consider now the car is traveling at full speed and that the patternpotentiometer PP fails as an open circuit. With the motor turning atfull speed, the tachometer generator is generating a sizable signaltending to oppose the pattern signal. Although it is desirable to bringthe speed of the car quickly under control under these conditions,application of the full voltage of the tachometer at this point wouldcause a severe jolt to the system. However, with the large resistor R31now in series with the tachometer generator a great deal of thetachometer generator signal is attenuated in the resistor R31 thereforeresulting in the application of a moderate signal to the SCR amplifiertending to reduce the speed of the car. Similarly, should the tachometergenerator fail as an open circuit while the car is traveling at fullspeed the loss of the retarding portion of the error signal would resultin a very large signal being applied to the SCR amplifier tending toincrease the speed of the motor. However the tachometer generator isshunted by a circuit including the armatures of the motor and thegenerator and the resistor R36. Although as mentioned previously theresistor R36 is very large, on the order of 660 K. while the resistorsR25 and R29 are only 1% K. and the effective resistance of the patternpotentiometer at maximum speed is only on the order of l K., the voltageacross the motor is much larger than the pattern voltage or the biasvoltage so that the feedback signal applied through the resistor R36exceeds the sum of the bias voltage and the pattern voltage therebyresulting in a resultant voltage which tends to bring the car to a slowspeed. It can be seen therefore that should either the patternpotentiometer or the tachometer generator fail as an open circuit thecar is smoothly brought to a slow speed. The automatic slowdown isfail-safe in that no switching is required to effect the slowdown.

We claim as our invention:

1. A variable-voltage system comprising a direct current motor, a directcurrent generator, connections for energizing said motor in accordancewith the voltage output of the generator, and a source of fieldexcitation for said generator including first signal-generating meansfor generating a signal attenuate to the desired speed of said motor,second signalgenerating means for generating a signal proportional tothe actual speed of said motor, said signal-generating means beingconnected in series opposition to generate an error signal,

means responsive to said error signal for supplying the field excitationfor said generator as a function of the error signal, and first shuntingmeans shunting one of said signal-generating means, said shunt meanshaving an impedance many times the impedance of said onesignal-generating means, whereby said shunt means is short circuited bysaid one signal-generating means under normal conditions and serves toattenuate the component of the error signal produced by the othersignalgenerating means when said one signal-generating means fails as anopen circuit.

2. The system of claim 1 including second shunting means shunting saidother signal-generating means and having an impedance many times theimpedance of said other signalgenerating means whereby should eithersignal-generating means fail as an open circuit the component of theerror signal produced by the functioning signal-generating means will beattenuated.

3. The system of claim 1 wherein said one generating means is the secondsignal-generating means and wherein said first shunting means shuntingsaid second signal-generating means includes the armature circuit ofsaid motor whereby the speed of the motor is smoothly reduced to a safevalue when said second signal-generating means fails as an open circuit.

4. The system of claim 3 wherein second signal-generating means is atachometer generator.

5. In an elevator system, a structure having a plurality of landingsincluding a terminal landing, an elevator car mounted for movementrelative to the structure to serve the landings, and motive means forcontrolling the movement of said elevator car between the landings, saidmotive means including, a motor connected to the car, a patterngenerator having a deceleration mode for generating a speed referencesignal to control the speed of the motor as a function of thedisplacement of the car from a landing at which the car is to bestopped, detector means operative from a first to a second condition asthe car approaches the terminal landing, an auxiliary speed referencesignal generator operative to apply a slow-speed reference signal to themotor in response to the operation of said detector means to said secondcondition when said pattern generator is not in said deceleration modeas the car approaches the terminal landing, and limit means operativewhen the car arrives substantially adjacent the terminal landing undercontrol of the auxiliary speed reference signal generator to stop themotor.

6. The system of claim 5 including means responsive to the operation ofsaid detector means to the second condition when said pattern generatoris not in the deceleration mode to prevent the restarting of the motorafter the motor has been stopped by said limit means.

7. A variable damped pattern generator for generating a speed referencesignal in a motor speed control comprising a variable tapped resistor,electrical supply means for applying an electrical voltage across saidresistor, means for varying the position of the variable tap inaccordance with desired changes in the speed reference signal, a fixedtap on the resistor, output means connected to the variable tap and thefixed tap on the resistor, and capacitive reactance shunted across theoutput means to form an RC filter with the variable resistance acrossthe output means whereby appreciable damping of variations in the speedreference signal is provided at high-speed settings of the variable tapwhen the resistance across the output means is substantial whilenegligible damping is imposed at low-speed settings of the variable tap.

8. The variably damped pattern generator of claim 7 in combination withan elevator speed control system comprising an elevator car mounted formovement to serve a plurality of landings, a motor for effecting themovement of the elevator car, and energizing means for energizing themotor in accordance with the speed reference signal generated by thepattern generator, the electrical supply means of said pattern generatorincluding means to apply voltages of opposite polarity to the oppositeends of said resistor whereby the pattern generator will supply a signalof a first polarity to the energizing means to cause the elevator car tomove in the up direction when the variable tap of the resistor isbetween the fixed tap and one end of the resistor and will supply asignal of the opposite polarity to the energizing means to cause themotor to move the elevator car in the down direction where the variabletap is between the fixed tap and the opposite end of the resistor 9. Inan elevator system, a structure having a plurality of landings, a carmounted for movement relative to the structure to serve the landings,door means on the elevator car for controlling the passage between theelevator car and any of the landings at which the car is stopped,door-operating means for opening and closing the door means, controlmeans for controlling the speed of the car as a function of thedisplacement of the car relative to a landing from which the car isstarted and relative to a landing at which a stop is to be made,preopening means responsive to the control means for causing thedoor-operating means to begin opening the door means as the carapproaches a landing at which a stop is to be made, limiting meansoperative to limit the speed attainable under control of the speedcontrol means when the car is started from a point other than a pointsubstantially adjacent a landing, and means responsive to the operationof the limiting means to render the preopening means ineffective tocause the door-operating means to begin opening the door means as thecar approaches a landing at which a stop is to be made.

10. In a variable-voltage system including a direct current motor, adirect current generator, connections for energizing said motor inaccordance with the output of said generator, control means forcontrolling the speed of the motor through variable energization of thegenerator field winding in accordance with an error signal, error signalgenerating means operative to generate a signalrepresentative of thedifference between a desired speed and the actual speed of the motorconnected across the inputs of the control means and noncontinuousnegative feedback means connected between the motor armature and theinputs of said control means in opposition to said error signal, saidnoncontinuous negative feedback means comprising switching meansoperative to block the passage of current until the voltage across saidswitch exceed's a predetermined value, a capacitor connected between theswitching means and one input of the control means and an impedanceconnected between the switch means and the second input terminal of thecontrol means, whereby the response of the motor to variations in theerror signal is clamped only above speeds at which the voltage acrossthe motor armature exceeds said predetermined voltage.

11. The system of claim 10 wherein said switching means is operative toblock the passage of current through the noncontinuous negative feedbackmeans in either direction until the voltage across the switching meansin either direction exceeds the predetermined voltage, whereby thenoncontinuous negative feedback means is operative to dampen theresponse of the system above a predetermined speed for either directionof motor rotation.

12. The system of claim 11 wherein said switching means comprises twoZener diodes each with a Zener breakdown voltage equal to saidpredetermined voltage connected backto-back in series in saidnoncontinuous negative feedback means.

1. A variable-voltage system comprising a direct current motor, a directcurrent generator, connections for energizing said motor in accordancewith the voltage output of the generator, and a source of fieldexcitation for said generator including first signal-generating meansfor generating a signal attenuate to the desired speed of said motor,second signal-generating means for generating a signal proportional tothe actual speed of said motor, said signal-generating means beingconnected in series opposition to generate an error signal, meansresponsive to said error signal for supplying the field excitation forsaid generator as a function of the error signal, and first shuntingmeans shunting one of said signal-generating means, said shunt meanshaving an impedance many times the impedance of said onesignal-generating means, whereby said shunt means is short circuited bysaid one signal-generating means under normal conditions and serves toattenuate the component of the error signal produced by the othersignal-generating means when said one signal-generating means fails asan open circuit.
 2. The system of claim 1 including second shuntingmeans shunting said other signal-generating means and having animpedance many times the impedance of said other signal-generating meanswhereby should either signal-generating means fail as an open circuitthe component of the error signal produced by the functioningsignal-generating means will be attenuated.
 3. The system of claim 1wherein said one generating means is the second signal-generating meansand wherein said first shunting means shunting said secondsignal-generating means includes the armature circuit of said motorwhereby the speed of the motor is smoothly reduced to a safe value whensaid second signal-generating means fails as an open circuit.
 4. Thesystem of claim 3 wherein second signal-generating means is a tachometergenerator.
 5. In an elevator system, a structure having a plurality oflandings including a terminal landing, an elevator car mounted formovement relative to the structure to serve the landings, and motivemeans for controlling the movement of said elevator car between thelandings, said motive means including, a motor connected to the car, apattern generator having a deceleration mode for generating a speedreference signal to control the speed of the motor as a function of thedisplacement of the car from a landing at which the car is to bestopped, detector means operative from a first to a second condition astHe car approaches the terminal landing, an auxiliary speed referencesignal generator operative to apply a slow-speed reference signal to themotor in response to the operation of said detector means to said secondcondition when said pattern generator is not in said deceleration modeas the car approaches the terminal landing, and limit means operativewhen the car arrives substantially adjacent the terminal landing undercontrol of the auxiliary speed reference signal generator to stop themotor.
 6. The system of claim 5 including means responsive to theoperation of said detector means to the second condition when saidpattern generator is not in the deceleration mode to prevent therestarting of the motor after the motor has been stopped by said limitmeans.
 7. A variable damped pattern generator for generating a speedreference signal in a motor speed control comprising a variable tappedresistor, electrical supply means for applying an electrical voltageacross said resistor, means for varying the position of the variable tapin accordance with desired changes in the speed reference signal, afixed tap on the resistor, output means connected to the variable tapand the fixed tap on the resistor, and capacitive reactance shuntedacross the output means to form an RC filter with the variableresistance across the output means whereby appreciable damping ofvariations in the speed reference signal is provided at high-speedsettings of the variable tap when the resistance across the output meansis substantial while negligible damping is imposed at low-speed settingsof the variable tap.
 8. The variably damped pattern generator of claim 7in combination with an elevator speed control system comprising anelevator car mounted for movement to serve a plurality of landings, amotor for effecting the movement of the elevator car, and energizingmeans for energizing the motor in accordance with the speed referencesignal generated by the pattern generator, the electrical supply meansof said pattern generator including means to apply voltages of oppositepolarity to the opposite ends of said resistor whereby the patterngenerator will supply a signal of a first polarity to the energizingmeans to cause the elevator car to move in the up direction when thevariable tap of the resistor is between the fixed tap and one end of theresistor and will supply a signal of the opposite polarity to theenergizing means to cause the motor to move the elevator car in the downdirection where the variable tap is between the fixed tap and theopposite end of the resistor.
 9. In an elevator system, a structurehaving a plurality of landings, a car mounted for movement relative tothe structure to serve the landings, door means on the elevator car forcontrolling the passage between the elevator car and any of the landingsat which the car is stopped, door-operating means for opening andclosing the door means, control means for controlling the speed of thecar as a function of the displacement of the car relative to a landingfrom which the car is started and relative to a landing at which a stopis to be made, preopening means responsive to the control means forcausing the door-operating means to begin opening the door means as thecar approaches a landing at which a stop is to be made, limiting meansoperative to limit the speed attainable under control of the speedcontrol means when the car is started from a point other than a pointsubstantially adjacent a landing, and means responsive to the operationof the limiting means to render the preopening means ineffective tocause the door-operating means to begin opening the door means as thecar approaches a landing at which a stop is to be made.
 10. In avariable-voltage system including a direct current motor, a directcurrent generator, connections for energizing said motor in accordancewith the output of said generator, control means for controlling thespeed of the motor through variable energization of the generatoR fieldwinding in accordance with an error signal, error signal generatingmeans operative to generate a signal representative of the differencebetween a desired speed and the actual speed of the motor connectedacross the inputs of the control means and noncontinuous negativefeedback means connected between the motor armature and the inputs ofsaid control means in opposition to said error signal, saidnoncontinuous negative feedback means comprising switching meansoperative to block the passage of current until the voltage across saidswitch exceeds a predetermined value, a capacitor connected between theswitching means and one input of the control means and an impedanceconnected between the switch means and the second input terminal of thecontrol means, whereby the response of the motor to variations in theerror signal is damped only above speeds at which the voltage across themotor armature exceeds said predetermined voltage.
 11. The system ofclaim 10 wherein said switching means is operative to block the passageof current through the noncontinuous negative feedback means in eitherdirection until the voltage across the switching means in eitherdirection exceeds the predetermined voltage, whereby the noncontinuousnegative feedback means is operative to dampen the response of thesystem above a predetermined speed for either direction of motorrotation.
 12. The system of claim 11 wherein said switching meanscomprises two Zener diodes each with a Zener breakdown voltage equal tosaid predetermined voltage connected back-to-back in series in saidnoncontinuous negative feedback means.